This invention relates to a gas manifold for a fuel cell such as a molten carbonate fuel cell in which the gas manifold supplies reactive gas to fuel cell stacks of the fuel cell and exhasuts reacted gas therefrom, and more particularly to an improvement of insulating properties of a connecting portion of the gas manifold which is installed on the side of the fuel cell stack.
FIG. 1 is a partially cutaway perspective view showing a prior art structure of a molten carbonate fuel cell stack which is disclosed, for example, in a DOE Report, SAN/11304-15, pp. 144-145. In FIG. 1, a fuel cell stack 1 is assembled in a box-like shape by a plurality of laminated single fuel cells 2, a pair of end plates 31 and 32 as auxiliary members, and a plurality of separators 4.
The single fuel cell 2 is composed of a fuel gas side electrode 5, an oxidant gas side electrode 6, and an electrolyte layer 7 which is sandwiched therebetween. Gas channels 51 and 61 are formed in the electrodes 5 and 6 for supplying a reactive gas to the electrodes 5 and 6, respectively. The gas channels 51 and 61 communicate to the inside of a gas manifold 8 described below.
A plurality of, for example four, gas manifolds 8 are connected on the side of the fuel cell stack 1 as shown in FIG. 1. Some of these gas manifolds 8 distribute fuel gas and oxidant gas to the electrodes 5 and 6 through the gas channels 51 and 61, respectively. At the electrodes 5 and 6 electrochemical reactions occur with the distributed fuel gas and oxidant gas. The reacted fuel gas and oxidant gas are exhausted from the electrodes 5 and 6 through the gas channels 51 and 61, and then gathered in the other manifolds 8, respectively. The gathered gases are then exhausted to the exterior of the fuel cell stack 1. In FIG. 1, arrow A shows the direction of the fuel gas flow, and arrow B the direction of the oxidant gas flow.
A gasket 9 is provided on a connecting portion of the gas manifold 8 for preventing a short-circuit between the single fuel cells 2 through the gas manifold 8. The gas manifold 8 is attached to the side of the fuel cell stacks 1 through the gasket 9. Each of the gas manifolds 8 has a flat-shaped casing 10 composed of metal which is open at one side and closed at the opposite side. A port 10a is provided near the center of the closed side of the casing 10 for supplying or exhausting gases.
The gas manifold 8 is assembled to the airtight against the fuel cell stack 1 through the open side of the casing 10 as shown in FIG. 2. In FIG. 2, the edges of the rectangular open end of the casing 10 are covered with an electric insulating layer 11. The casing 10 is then assembled to the side of the fuel cell stack 1, sandwiching the insulating layer 11 between the casing 10 and the gasket 9.
A molten carbonate type fuel cell, for example, is a type of fuel cell which operates at temperatures around 650.degree. C. This type of fuel cell converts the chemical energy of the fuel gas to electrical energy and by-productive heat energy. This conversion of energy is achieved by electrochemical reactions at the electrodes 5 and 6 with the fuel gas and oxidant gas, respectively. Therefore, in order to operate the fuel cell steadily and to produce electrical output from the fuel cell, it is necessary to continuously supply the reactive gases to the electrodes 5 and 6, and also to continuously exhaust the reacted gases from the electrodes 5 and 6.
In FIG. 1, to achieve the above supply and exhaust of gases, the reactive gases are supplied, using the gas manifold 8, to the electrodes 5 and 6 through the gas channels 51 and 61, respectively. Then, the reacted gases at the electrodes 5 and 6 are gathered and exhausted through the other gas manifolds (not shown), respectively.
In supplying the reactive gas and exhausting the reacted gas of a fuel cell having the structure as shown in FIG. 1, one of the most difficult technical points is the selection of the material and structure of the gasket 9 itself and the portions adjacent to the gasket 9. Firstly, the gasket 9 itself must have sufficient insulating properties to prevent short-circuits between the laminated upper and lower sides of the single fuel cells 2 through the gasket 9, and between the single fuel cells 2 through the gasket 9 and the casing 10. Secondly, during operation of the fuel cell sufficient corrosion resistance is required of the gasket 9 itself, the portions of the casing 10 adjacent to the gasket 9, and portions of the fuel cell stack 1 adjacent to the gasket 9. If the gasket 9 and the other components adjacent thereto do not have sufficient corrosion resistance, products of corrosion may be deposited in voids of the gasket 9 for long periods of operational time of the fuel cell. As a result of such corrosion products, short-circuits could be caused on the single fuel cells 2 themselves or between the single fuel cell 2 and the casing 10 of the gas manifold 8, making it impossible to operate the fuel cell.
For the purposes of preventing short-circuits, an insulating layer 11 composed of an electric insulating material is coated directly on to the end fringe portion of the opening of the casing 10. This layer 11 is for insulating the fuel cell stack 1 from the casing 10 and for preventing the single fuel cells 2 from short-circuiting through the gasket 9 and the casing 10.
The gasket 9 is fabricated from a porous material composed of an inorganic insulating material which has a slight ionic conductivity for the purpose of holding the electrolyte, which has oozed from the electrolyte layer 7, into the voids of the gasket 9. However, short-circuits between the single fuel cells 2 through the gasket 9 are substantially prevented by increasing the electric resistance of the gasket 9. The increase of the electric resistance is achieved by decreasing the void volume of the gasket 9 or changing the shape of the voids of the gasket 9.
The conventional fuel cell having the gasket 9 described above operates favorably for short periods of time. However, since the insulating layer 11 is coated directly on the end portion of the casing 10, for long periods of operation time there arises the following problems, which are mentioned in the DOE Report SAN/11304-15.
The voids in the insulating layer are initially formed from defects that occur during the formation of the insulating layer 11. The electrolyte then permeates into such voids, thus hastening corrosion of the casing 10. Moreover, metal oxides are formed by the corrosion, and accumulate in the voids of the gasket 9, resulting in short-circuits between the single fuel cells 2 or between the single fuel cells 2 and the casing 10.
Secondly, since the fuel cell is operated at temperatures around 650.degree. C., cracking occurs at the insulating layer 11 as a result of thermal stress arising from the differences of thermal expansion coefficients between the insulating layer 11 and the casing 10 of the gas manifold 8. Such cracks can deteriorate the insulating properties of the insulating layer 11 itself, and the corrosion of the casing 10 of the gas manifold 8 would be accelerated by any electrolyte which permeates into the cracks. As the cracks progress, metal oxides are produced as above, resulting in short-circuits between the single fuel cells 2 or between the single fuel cells 2 and the gas manifold 8.
In the conventional gas manifold 8 as shown in FIG. 2, short-circuits due to the above-mentioned reasons occur within several hundred to several thousand hours of fuel cell operation. In addition, electrolyte penetrating the cracks through the insulating layer corrodes the casing surface beneath the insulating layer. As a consequence, the insulating layer peels away from the corroded casing surface. Therefore, one of the important areas for developing fuel cell stacks relates to solving the above problems.
In the conventional gas manifold constructed as above, it is difficult to maintain the insulation between the single fuel cells 2 and between the single fuel cells 2 and the gas manifold 8 for long periods of time, because of the corrosion at the casing 10 of the gas manifold 8 and the cracks arising in the insulating layer 11 from thermal stress. For the same reasons, the fuel cell cannot be stably operated for long periods of time.